The present invention relates generally to a projection exposure apparatus used for the photolithography process, and more particularly to an exposure dose control in the scanning exposure apparatus.
A projection exposure apparatus that employs a projection optical system to expose a reticle (mask) pattern onto a wafer has been conventionally used. A scanning exposure apparatus has recently been reduced to practice for wide-screen exposure. The precision required for a pattern's critical dimension (“CD”) uniformity becomes stricter as the fine processing develops.
In order to reduce the uneven illuminance on the screen that aggregates the CD uniformity, the conventional scanning exposure apparatus controls the exposure dose using a light blocking plate near a scan masking blade that defines an illumination area or slit. For example, the exposure dose around the light blocking plate is set to be larger when the optical system has a higher transmittance at the center of the optical axis than at the periphery.
Prior art includes, for example, Japanese Patent Application, Publication No. 9-190969, and CD Uniformity Improvement by Active Scanner Corrections, Jan Van Schoot et al., Proceedings of SPIE, Vol.4691, SPIE, 2002, pp.304-314 (“Schoot” hereinafter).
The conventional exposure apparatus corrects a CD error caused mainly by the exposure apparatus, as in Japanese Patent Application, Publication No. 9-190969. The conventional exposure apparatus adjusts the uneven illuminance in a direction corresponding to a direction orthogonal to a scanning direction using a slit width adjusting mechanism. The slit width adjusting mechanism generally includes a mechanical blade, adjusts the uneven illuminance whenever an illumination mode and a numerical aperture (“NA”) of the projection optical system are varied. The uneven illuminance among wafers and shots have not been corrected.
However, as suggested by Schoot above, it has recently been required to correct a deterioration of the CD error caused by processes and apparatuses other than the exposure apparatus, such as a coater, a developer, an etcher, and a reticle imaging apparatus. Schoot discloses means for correcting the uneven illuminance by moving, in the scanning direction, a gray filter that has a different distribution according to positions. Nevertheless, this means requires a driving mechanism similar to the reticle stage to drive the filter in the scanning direction and to correct the uneven illuminance, and would cause in a large exposure apparatus and an increased cost.